Arrays of capacitive proximity sensors are often used in keyboards, keypads and other touch-input apparatus. Two characteristics of capacitive sensors that lead to their use instead of conventional metallic switches are: 1) Capacitive sensors can be made in small sizes, which is desirable if a small, tightly packed keyboard is required; and 2) Capacitive sensors are particularly easy to environmentally seal, which is desirable if the keyboard is to be used in a wet environment or where there is a concern that contaminants may be spilled on the keyboard.
Conventional capacitive sensors, when tightly packed, or when used in the presence of conductive liquid films, suffer a keying ambiguity problem. In a small keyboard, for example, a user's finger is likely to overlap from a desired key onto adjacent ones. This is especially problematic if the user has large fingers or if he or she presses on the keyboard surface hard enough to deform his or her finger. The same sort of effect is found when a conducting film is spilled on a keyboard, in which case the user's finger is sensed as though it were the size of the puddle. Problems of this sort are particularly acute in cash register keyboards used in food service establishments where beverage and food sauce spills are a frequent occurrence.
In his U.S. Pat. No. 5,730,165, the inventor teaches a capacitive field sensor employing a single coupling plate, which may be used as a key switch, and a method of detecting a change in capacitance of the coupling plate to ground. The apparatus taught in U.S. Pat. No. 5,730,165 comprises pulse circuitry for charging the coupling plate and for subsequently transferring the charge from the plate into a charge detector, which may be a sampling capacitor. The transferring operation is carried out by means of a transfer switch electrically connected between the coupling plate and the charge detector. One aspect of the teaching of that patent is the provision of apparatus for measuring a capacitance to ground of a plate, the apparatus comprising: a control means; a charging means connecting the plate to a voltage source supplying a first selected voltage; a discharging means responsive to a discharging output from the control means to connect the plate to a charge measurement capacitor that has a selected capacitance value and that has a first of its two terminals maintained at a second selected voltage, the second of the terminals supplying a voltage output representative of the capacitance to ground of the plate. From another viewpoint, U.S. Pat. No. 5,730,165 teaches a method of operating a capacitive sensor for sensing the presence of an object proximate a sensing plate by measuring a change in a capacitance to an electrical ground. This method comprises the steps of: a) setting an output of a charge detector comprising filter means to a first selected voltage; b) charging the plate from a voltage source to a second selected voltage; c) discharging, for a selected discharging interval, the plate into the charge detector; d) repeating steps b) through c); and e) reading an output of the charge detector as a representation of the capacitance of the plate. Yet another provision of U.S. Pat. No. 5,730,165 is a method of operating a capacitive sensor electrically connected between an electrical ground and a sensing plate disposed adjacent an external surface of a first object (e.g., a keyboard), the sensor measuring a capacitive change responsive to a second object (e.g., a user's finger) adjacent the first object, where the two objects are separately electrically coupled to the ground, the method comprising the steps of: a) charging the plate from a voltage source; b) connecting, for a discharging interval having a selected discharging duration, the plate to a charge detector having an output representative of the change of the capacitance to ground of the plate; and c) reading the output of the charge detector. The disclosure of U.S. Pat. No. 5,730,165 is herein incorporated by reference.
In his U.S. Pat. No. 6,466,036, the inventor teaches pulse circuitry for measuring capacitance to ground, the circuitry comprising a plurality of electrical switching elements, each of which has one side electrically connected to either a power supply voltage or to a circuit ground point. This circuit arrangement, which may be used with a keyboard as well as for many other applications, is more compatible with available integrated circuit design and manufacturing practices than is earlier pulse circuitry, which commonly had one side of at least one switching element floating. These improved arrangements provide superior performance at a lower manufacturing cost.
In his U.S. Pat. No. 6,466,036, the inventor describes apparatus for measuring the capacitance to ground of a plate. This apparatus comprises at least three switching elements, each of the at least three switching elements having both a respective open state and a single respective closed state, each of the at least three switching elements electrically connected to one of two distinct reference voltages; a sample capacitor having a proximal one of its two terminals connected to the plate by means not comprising one of the switching elements; a voltage measurement circuit connected to one of the terminals of the sample capacitor by means not comprising one of the switching elements, the measurement circuit comprising one of a logic gate and a voltage comparator. In this arrangement a first of the at least three switching elements connects the distal terminal of the sample capacitor to the first reference voltage when in its closed state and disconnects the distal terminal from the first reference voltage when in its open state. A second of the at least three switching elements connects the proximal terminal of the sample capacitor to the second reference voltage when in its closed state and disconnects the proximal terminal from the second reference voltage when in its open state. The third of the at least three switching elements connects the distal terminal of the sample capacitor to the second reference voltage when in its closed state and disconnects the distal terminal from the second reference voltage when in its open state. The apparatus further comprises a controller for operating the at least three switching elements so that at any time at least one of the at least three switching elements is in its respective open state. The disclosure of U.S. Pat. No. 6,466,036 is herein incorporated by reference.
In his U.S. Pat. No. 6,452,514, the inventor teaches several key switch arrays based on measuring changes in capacitive coupling between electrode pairs. In particular, he teaches a key matrix comprising a first plurality, N, of input lines, a second plurality, M, of output lines and N times M keys, wherein each key comprises a respective capacitive sensor for sensing the presence of an object proximate thereto and wherein each key is disposed adjacent a respective intersection of an input line and an output line. This key matrix is characterized in that each of the keys comprises a respective driven plate connected to one of the N input lines and a respective receiving plate connected to one of the M output lines; each of the input lines is electrically connected to a respective signal generator for supplying a respective periodic plurality of voltage cycles to each of the driven plates connected to it. There is also a controller for controlling each of a plurality of switching elements in an arrangement in which it controls a first selected number of the plurality of switching elements so as to connect each of the receiving plates to a reference potential when that receiving plate is not connected to one of the two terminals of a charge integrator that can be selectively connected to at least one of the receiving plates during a selected portion of the respective voltage cycle supplied to the driven plate associated with the at least one of the receiving plates, the charge integrator directly connected to at least a first of the plurality of switching elements. There is also a measurement circuit for measuring a voltage on the charge integrator as an indication of the proximate presence of the object. The disclosure of U.S. Pat. No. 6,452,514 is incorporated herein by reference.